1. Field of the Invention
This invention relates to a method of detecting a reduction in the degree of vacuum of a vacuum valve which is employed in a vacuum circuit breaker or vacuum contactor, and a device for practicing the method.
2. Description of the Related Art
FIG. 6 is a side view showing a vacuum circuit breaker. The circuit breaker comprises a vacuum valve 1 with a stationary lead 7, and a movable lead 8. The movable lead 8 is connected through an insulating rod 20 to one end of an operating lever 21, the other end of which is coupled to an operating unit (not shown) which is accommodated in an insulating frame 13. The movable lead 8 is further connected through a flexible wire 22 to a main circuit terminal 23. The stationary lead 7 is connected to another main circuit terminal 24. Those main circuit terminals 23 and 24 are connected to insulators 25 and 26, respectively. The vacuum circuit breaker is of pull-out type, and the lower end portion of the insulating frame 13 is mounted through wheels 27 on the frame 28 of a power board. That is, when the vacuum circuit breaker is moved right and left in FIG. 1, clips 23A and 24A connected to the ends of the main circuit terminals 23 and 24 are connected to and disconnected from the main circuit bus lines (not shown).
FIG. 7 is a sectional view showing essential components of the vacuum circuit breaker shown in FIG. 6. In the vacuum valve 1, the stationary electrode 2 and the movable electrode 3 form contact means, and an arc shield 4 surrounds the contact means. The contact means and the arc shield 4 are built in a cylindrical insulating container 5. The container 5 is sealingly closed with lids 6A and 6B at both ends, and the inside of the container is evacuated high in the degree of vacuum. The stationary electrode 2 is fixedly secured to the bottom 6A, and it is electrically connected through the stationary lead 7 to an external circuit. The arc shield 4 is fixedly mounted on the lid 6A. The arc shield 4 is to prevent the metal vapor from sticking onto the inner cylindrical surface of the insulating container 5 which is produced by arcs formed at the contact means when the circuit is shut off. In the vacuum valve 1 shown in FIG. 7, the arc shield 4 is mounted on the lid 6A; however, it may be mounted on the lid 6B, or it may be provided at the middle of the container 5 in such a manner that it is insulated from the two lids 6A and 6B. FIG. 7 shows the vacuum valve with the contact means closed.
The vacuum valve 1 is hermetically sealed so that the vacuum pressure in the insulating container 5 is smaller than 10.sup.-4 Torr. If the sealing of the insulating container is impaired; that is, if the vacuum pressure in the insulating container 5 is raised, the vacuum valve 1 is lowered in breaking characteristic and in insulating characteristic, and therefore it is no longer useful. Thus, detection of the degree of vacuum of the vacuum valve 1 is essential for maintenance of the vacuum circuit breaker.
Heretofore, in order to detect the degree of vacuum of the vacuum valve thereby to determine whether or not the sealing of the latter is satisfactory, high voltage is applied across the contact means which is held open; that is, a withstand voltage test is given to the latter. This method is based on the fact that the flashover voltage of the contact means is decreased as the vacuum pressure decreases.
However, the above-described conventional method is disadvantageous in that, in order to test the vacuum valve, the latter must be disconnected from the main circuit. More specifically, in order to apply high voltage to the vacuum valve, the vacuum valve must be disconnected from the main circuit by pulling it out of the power board. Hence, the maintenance takes lots of time and labor, and the electric current is cut off for a long period of time.
The degree of vacuum of the vacuum valve may be measured with a vacuum meter connected to it. However, this method suffers from the following difficulties: As was described above, it is essential to maintain the degree of vacuum of the vacuum valve high. Hence, in connecting the vacuum meter to the vacuum valve under test, it is necessary to positively prevent the sealing of the vacuum valve from being impaired at the connecting point of the vacuum meter. At worst, the connection of the vacuum meter to the vacuum valve may lower the reliability of the latter. Thus, the method of measuring the degree of vacuum with the vacuum meter is not practical.